In rare occasions, earlier in geologic history when uranium-235 was more abundant, uranium ore was found to have naturally engaged in fission, forming natural nuclear fission reactors. Uranium-235 decays at a faster rate (half-life of 700 million years) compared to uranium-238, which decays extremely slowly (half-life of 4.5 billion years). Therefore a billion years ago, there was more than double the uranium-235 compared to now.

During the Manhattan Project, the name Tuballoy was used to refer to natural uranium in the refined condition; this term is still in occasional use. Uranium was also called codenamed "X-Metal" during World War II.